Accounting for scientific uncertainty in a dangerously warming world

Most people at some point develop a “Plan B” – in case their first choice of college doesn’t accept them, or it rains on the day of their planned outdoor party, or the deal for the house they wanted falls apart. The same principle applies for more dire situations, such as a city having plans in hand for an orderly evacuation in case of a large-scale disaster. We hope such an event will never happen, but the mayor had better be prepared in case it does.

In a commentary today in the scientific journal Nature Climate Change, three colleagues and I discuss the need for a “Plan B” for climate change: How will we cope with increasingly severe climate impacts if we are unsuccessful in limiting global warming to a chosen target?

In the 2009 Copenhagen climate accord, countries set a goal of limiting global warming to below 2 °C (3.6 °F) above the average global temperature of pre-industrial times. However, given that the planet has already warmed by 0.8 °C, additional warming is already locked into the system, and global greenhouse gas emissions continue to rise, this “Plan A” has become increasingly difficult and may become impossible to achieve if widespread emissions reductions do not begin within this decade. A maximum warming target is a necessary goal of climate policy, but what if our efforts fall short?

Some voices in the environmental community will feel that asking this question is ceding failure, but I disagree. Instead, it means admitting that we can’t perfectly foresee the future and that we need to be prepared for surprises. This is called risk management and everyone from parents, to mayors, to companies, to the U.S. military uses risk management every day to cope with uncertainty.

Scientists have offered their best estimates of how the climate system is likely to respond to warming and what impacts are likely. Since there is no crystal ball for the future however, scientists have also carefully documented the associated uncertainties (p 25). Often, these uncertainties are exploited to raise doubts that climate change is a problem or ignored to avoid raising such doubts. Both reactions are misguided. Uncertainty itself is what creates the risk and is itself the information required to develop a risk management plan for climate security. And fundamental science has shown that the current uncertainties are skewed such that climate impacts are more likely to be underestimated than overestimated (p. 35).

Because of these uncertainties, there are several ways climate policy could fall short of its goals. For example, even if we agree on and then achieve stringent emissions targets intended to limit warming to 2 °C, it could turn out that the targets were miscalculated and we end up with more warming than anticipated. Or we could succeed at limiting warming to 2 °C, but find out this level of warming is far more dangerous than scientists understood. Uncertainty extends to policy implementation as well: We could have the correct technical goals in mind but find that the policies implemented were not effective in achieving them. All of these scenarios succeed in some way initially, yet ultimately fail to secure society from the impacts of climate change. This is why it is crucial that local, national, and international policies account for the uncertainties – in both science and policy – even as we aim to limit warming to a particular target.

Contingency plan in case of vastly underestimating warming or its consequences

Plan A is what the international policy community is already attempting to do, but there is very little discussion of B or C.

Building and planning for more warming than 2 °C simply means that when making long-lived investments, we should weigh the cost of early loss/retirement against the cost of adaptations to withstand climate impacts. Some adaptations (e.g., a different siting decision) might not be more expensive but simply would not be considered under the assumption that warming will not exceed 2 °C. Where extra costs would be incurred (e.g., hardening or relocating infrastructure), they might be considered insurance against large potential losses in the long term.

Contingency planning was previously recommended by Vice Adm. Richard Truly as a member of a blue ribbon panel of retired senior military officers who studied the implications of climate change for U.S. national security:

“The stresses that climate change will put on our national security … will affect every nation, and all simultaneously. This is why we need to study this issue now, so that we'll be prepared and not overwhelmed by the required scope of our response when the time comes." (National Security and the Threat of Climate Change, page 14)

For example, if conditions worsen so that society demands action to deal with the causes and consequences of climate change, what would be the least costly, most beneficial options? One or more countries might consider large-scale climate engineering (often called “geoengineering”), which might offer benefits but could also carry grave risks. The scientific knowledge needed to make these decisions doesn’t exist today. Another option might be early retirement of capital stock, such as fossil fuel power plants, to decarbonize the economy. If this course were adopted, how would we quickly replace that energy with more acceptable technologies? If millions of people living in coastal areas need to move out of harm’s way because of sea-level rise and larger storm surges, where would they go and how could they be accommodated? What if the world’s food supply is disrupted? We should be considering our responses to these potential outcomes long before they might be realized.

While we don’t want to end up in B or C territory, I would rather be prepared for those potential outcomes, even while hoping for Plan A to succeed. Moreover, seriously examining our options under B and C might clearly distill the benefits of Plan A.